In electronics devices, particularly consumer electronics audio/video players and recorders, there is a need to exchange digital data between devices. This is so that audio or video data can be exchanged for processing. For example, an audio amplifier typically has a digital input to receive an output from a DVD player. To provide such an interface various standards are known, one such being the known Sony Philips Digital InterFace format (SPDIF).
The SPDIF format is used for digital exchange of audio data between devices. The format is well known and is briefly described later for completeness. In brief, the format comprises a structure of 32 bit sub-frames with two such sub-frames (left and right audio) comprising a frame of data. The frequency of data can vary over a wide range (16 kHz-192 kHz), the higher the frequency, the better the quality of the audio signal. The signal is bi-phase encoded so that the clock signal and data signal are encoded on a single line.
There is a need to recover the clock signal from the data signal. There is also a need to identify the start of each data frame and sub-frame. We have appreciated that the characteristics of a data signal of differing possible frequencies can be used to determine and lock on the start of data frames and determine the unit interval on frequency of the signal. The invention is defined in the claims to which reference is now directed.
The invention makes use of the appreciation that the preamble section (a header) of a digital data frame or sub-frame can include certain characteristic markers by which the start of a sub-frame or frame can be determined, as well as the frequency of the signal. In the embodiment, the particular characteristic is the fact that the preamble of a sub-frame includes a pulse (in the sense of the time between the successive signal edges) that is longer than any such pulse in the data signal. This allows the position of the preamble and the frequency of the clock signal to be quickly determined.
The preferred embodiment of the invention relates to the SPDIF format of digital audio signal. In this format the preamble includes a portion which violates the normal bi-phase coding rules and, in consequence, has a gap between the successive edges that is 6 unit intervals (UI) in contrast to the longest gap of 4 unit intervals (UI) in the data portion of the data stream. The embodiment therefore includes a longest pulse detector that determines whenever there is a longer gap between edges than preceding bits. At this point, the output of the longest pulse detector is asserted, indicating to a signal decoder the position of the frames and sub-frames and the clock frequency.
The invention allows very fast locking of the position a and frequency of a binary phase encoded signal with a characteristic preamble or header. The embodiment counts for a period long enough that at least one sub-frame header would be seen (in the case of SPDIF this is 128 bits). The longest pulse received in that time is known to be a specific position in the preamble that is 6 UI. By counting every third period the clock period of 2 UI can be rapidly recovered.
The invention may apply to formats other than SPDIF, but is particularly applicable to digital signals having a given format, but wide possible range of signal frequency.